For quite a long period it has been conventional to employ a free-wheeling device in connection with, or as a part of, a rear wheel. This allows the rider to coast the cycle without continuing to turn the pedals. Freewheeling became a necessary part of the overall drive train design when multiple rear sprockets and a chain derailleur were employed to achieve multi-ratio speed selection.
Until the last ten years, the hub portion of the rear wheel was composed of two separate assemblies; the hub body, a shell with flanges and spoke holes, two bearings mounted inside, and an axle running through the middle; and a freewheel, composed of the multiple sprockets and a one way rotation device. The freewheel was attached to the hub by means of a standard threading, allowing for interchanging in case of repairs or replacements.
More recently, the freewheel and the hub body were integrated into one unit or "freehub". As a part of the one way device, this unit had a single, external splined diameter on the drive side of the hub by which a group or "cassette" of sprockets having a matching splined hole could be mounted. This arrangement allowed for much quicker changing of selectable gear ratios.
Whether it be the older screw-on type freewheel or the newer freehub, the one way device is conventionally of the ratchet and pawl design. This design has been the preferred type because of its positive engagement, and relatively simple and inexpensive construction. However, the small number of pawls that can fit into the confined space within the ratchet ring requires the turning energy to be transferred through a minimal amount of material and surface area. Because of this, it is essential that the pawls, pawl nest component, and the ratchet ring be made of hard steel. The coarseness of the teeth on the ratchet ring must also be maximized to maintain a maximum amount of force bearing surface for the pawls to push on once engaged. Both the weight of the steel pieces and coarseness of the ratchet teeth have become obstacles in the path of freewheel improvement.
Weight savings is commonly a paramount subject in bicycle design and thus needless to talk about here. However, the coarseness of the ratchet teeth is worth exploring. Through the course of bicycle history, the significance of how quickly a freewheeling device engages has never been realized. Until the development and popularization of the "Mountain Bike" or multi-terrain cycle, bicycling technology has been quite content with the engagements per turn of the typical pawl and ratchet freewheel. Today's off road cycling sports has quite different needs. The typical mountain bike has gearing or final drive ratios that are far lower than bicycles in the past. In its lowest gear, pedal movement of over 2 inches may be experienced while waiting for a typical pawl and ratchet freehub to engage. The importance of quickness of engagement is experienced while riding up a steep trail and having to reposition your pedals to miss hitting a rock or root and then continue to pedal to keep forward momentum. Having a freehub that engages more rapidly improves the connection of pedal power to the rear wheel thus not losing precious momentum. Other advantages become clear when thinking about not losing time when connecting power to the rear wheel.
More recently, friction clutch devices such as pin-roller, ball, or sprag etc. were employed to take the place of the pawl and ratchet system. Although having the advantage of near instantaneous engagement, they suffer from a side effect of the clutch devices: while generating the necessary friction to transfer torque through them they produce very large amounts of radial force. In turn, this radial force must be contained within the assembly in order for the clutch to produce the necessary friction to transfer the torque. It is the containment of this radial force that condemns this design. Either the clutch has to be contained in, or made of, a large mass of heavy steel or the clutch will not be able to generate enough friction to handle the large amounts of torque generated by low final drive gearing.